I think when the clutch gets near to the end of its life.
on long journeys especially in summer the clutch components get hot and heavy,this may cause a seal on the hydrolics to fail.You pay out for a new slave & master cylinder but you still have a problem.Look for a glossy sticky mark on the sound insulation around the pedals.We just use our V reg grand Espace as a local runaround.Go too far and the gearchange is difficult. This is my opinion only.
M Dee, May 2009